This invention relates to apparatuses for gas chromatography and particularly to novel sample collection and inlet systems for such devices.
Gas chromatography is a widely employed technique for the separation and analysis of complex mixtures of volatile organic and inorganic compounds. The analyte mixture is separated into its components by eluting them from a column having a sorbent by means of a moving gas.
Gas chromatography procedures can be classified into two major divisions; gas-liquid chromatography, and gas-solid chromatography. Gas-liquid chromatography is presently the most widely used type and incorporates a non-volatile liquid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography column. The analyte partitions itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficient or solubility of the analyte components. The analyte is introduced at the entrance end of a column within the moving carrier gas stream. The components making up the analyte become separated along the column and escape from the exit end of the column at intervals and in concentrations characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the column responds to the presence of the analyte components. Upon combustion of the eluted material at a FID, charged species are formed in the flame. The flame characteristics are monitored through a biased ion detector which, along with associated electronics, produces a chromatogram which is a time versus magnitude trace of the detector output. The trace for complex mixtures includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of the chromatogram.
Gas chromatography systems of the type generally described above are in widespread use today. Although present systems provide excellent performance and utility, this invention seeks to provide a number of improvements to existing apparatuses. Various approaches are presently used for introducing a sample into the separation column. In one general type of gas chromatography systems, a thermal focusing chamber or cold trap is employed. The cold trap is typically a vessel containing a cold gas such as nitrogen having a capillary sample tube passing through it which conducts the analyte. By exposing incoming analyte to the low temperatures within a cold trap, the analyte components condense on the capillary tube. When it is desired to inject a sample into the column for separation, the temperature of the sample tube passing through the cold trap is increased rapidly thus vaporizing the sample. The carrier gas stream which continually flows through the trap then injects the analyte into the column for separation.
In a typical gas chromatography system of the type employing a thermal focusing chamber, during the trapping mode of operation, the incoming analyte is trapped at the inlet end of the cold trap sample tube (i.e. in the direction of carrier gas flow during injection). After heating the cold trap sample tube, the sample components must traverse the entire length of the sample tube before introduction into the column. The sample flow circuit regions between where the component is vaporized and the beginning of the column constitutes system "dead volume" which is undesirable since it results in broadening of the injected analyte in terms of the time duration over which it is presented at the inlet end of the column. Dead volume adversely affects system resolution and efficiency.
Today there is increased emphasis toward so called "fast gas chromatography" or "fast GC". Applications include process stream monitoring, environmental monitoring, and engine exhaust gas analysis. Ideally such systems would be able to perform an analysis within several seconds which previously took several minutes or more. Increasing the speed of analysis can be achieved by providing a relatively short separation column or by using other techniques for causing components of interest to traverse the column quickly. In order to provide useful information, the individual analyte components must elute separately at the detector, thus producing distinct peaks. As the length of time that the sample is injected at the inlet end of the column increases, the peaks produced by elution of the components tend to broaden, smear and overlap. It is, therefore, essential that a narrow sample "plug" be presented at the column during injection in order to provide gas chromatography evaluation in a small period of time. It is for this reason that the dead volume associated with conventional cold trap type gas chromatography systems is a disadvantage.
In gas chromatography systems of the type described previously which employ a thermal focusing chamber or cold trap, it must be understood that the entire length of the cold trap sample tube cannot be maintained ideally at a uniform constant temperature, either during the collection or injection modes. In fact, a temperature gradient exists at the inlet and outlet ends of the cold trap capillary tube. Since during the collection mode of operation, the analyte condenses at near the inlet end of the capillary tube (in terms of the direction of flow of carrier gas during injection), it is necessary to insure that that region is sufficiently heated to vaporize all of the components of interest of the mixture during the injection step. This requirement leads to some portions of the cold trap sample tube being heated to a significantly higher temperature than is necessary to vaporize the sample collected at the inlet end of the sample tube. Furthermore, the analyte is exposed to the excessive temperatures for the length of time necessary to conduct them entirely through the focussing chamber. These excessive temperatures and the significant "residence time" in the sample tube have been related to decomposition of analyte components. Accordingly, instead of components in their natural state being ejected from the column, these components become fragmented into parts of the initial molecule. Such decomposition of the sample significantly complicates analysis and can render the generated chromatogram of little value in certain types of evaluation.
Furthermore, in many conventional gas chromatography systems, mechanical valves are used to control the inlet flow of analytes. Valves are available which are especially designed for gas chromatography systems and are generally micro-pneumatic type valves which have a relatively small dead volume. Despite the advanced state of the design of present valves, they nonetheless contribute to dead volume and have a tendency to retain a small portion of a prior sample which becomes mixed with the following sample during the next actuation of the valve. Accordingly, prior samples can influence subsequent samples, creating an undesirable artifact termed a "memory effect".
Mechanical valves which are used in many present gas chromatography systems are used for conducting the sample flow. When using valves in this manner, inevitable sample loss occurs as the sample coats internal surfaces within the valves. Another disadvantage of such valves in the sample flow path is the fact that they can contaminate the sample with lubricants or other coatings which are present in the valve.
The first described embodiments of a gas chromatography system in accordance with the present invention improves over present devices with respect to each of the previously described areas. A significant reduction in inlet system dead volume and decomposition of the analyte during injection is achieved principally through a gas chromatography circuit which introduces the sample into the cold trap sample tube at the end which is closest to the separation column (i.e. the outlet end for the carrier gas during the injection mode). In other words, the system utilizes a reverse flow direction through the cold trap during the collection mode as compared with injection. Since the analyte is trapped directly adjacent the separation column, the retained volume between the point of collection and the column is minimized, thus reducing system dead volume. This trapping approach also provides another significant benefit; namely, that upon injection the analyte components are only heated to the degree necessary to vaporize them whereupon they exit the cold trap without passing through a positive temperature gradient, reducing decomposition. In fact, these inventors have found that satisfactory injection can be achieved without increasing the temperature of the cold trap sample tube to the level necessary with prior art systems, enabling the use of lower capacity power supplies. Perhaps more importantly, the lower injection temperatures significantly enhances the operational longevity of the cold trap sample tube which is subject to significant temperature extremes and gradients leading to mechanical stresses and eventual failure.
Another benefit of the system according to this invention is that the sample is introduced into the system and injected into the column without ever passing through mechanical valves. This advantage leads to virtual elimination of the memory effect and contamination discussed previously. The system according to the present invention is further adaptable for evaluating samples introduced at a broad range of ambient pressures for use as an air quality "sniffing" probe or with positive pressure inlet sources.
A gas chromatography system in accordance with a second embodiment of this invention does not provide the feature of trapping analyte at the outlet end of the cold trap sample tube. However, it does provide the features of eliminating mechanical valves in the sample flow path and in fact, requires only a single valve to control its operation. Thus the problems associated with wear and failure of mechanical valves are minimized in the system of the second embodiment of this invention. The system of the second embodiment is furthermore vastly simplified in terms of the number of components and connections which provides inherent advantages in terms of reliability.
Both the embodiments of this invention provide the additional benefit that the inlet system is continually flushed with carrier gas when collection is not taking place, thus further reducing the likelihood of the memory effect discussed previously.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.